THE ASTRONOMICAL JOURNAL, 115 :234È246, 1998 January1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

OPTICAL LIGHT CURVES OF THE TYPE Ia SUPERNOVAE SN 1990N AND SN 1991T

P. NICHOLAS B. M. M. AND MARIOLIRA,1 SUNTZEFF, PHILLIPS, HAMUY2Cerro Tololo Inter-American Observatory, National Optical Astronomy Casilla 603, La Serena, Chile ;Observatories,3

p.lira=roe.ac.uk, nsuntze†=noao.edu, mphillips=noao.edu, mhamuy=as.arizona.edu

JOSE� MAZA

Departamento de Astronom•� a, Universidad de Chile, Casilla 36-D, Santiago, Chile ; jmaza=das.uchile.cl

R. A. R. C. LISA A. R. J. A. J. H.SCHOMMER, SMITH,4 WELLS,2 AVILE� S, BALDWIN, ELIAS,5L. A. M. P. ALISTAIR R.GONZA� LEZ, LAYDEN,4,6 NAVARRETE, UGARTE, WALKER,

AND GERARD M. WILLIGER7Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories,3

Casilla 603, La Serena, Chile

F. K. BAGANOFF

Center for Space Research, Massachusetts Institute of Technology, 37-518A,77 Massachusetts Avenue, Cambridge, MA 02139

ARLIN P. S. R. MICHAEL AND N. D.CROTTS, RICH, TYSON8Department of Astronomy, Columbia University, Mail Code 5242, New York, NY 10027

A. DEY

Kitt Peak National Observatory, National Optical Astronomy Observatories,950 North Cherry Avenue, Tucson, AZ 85726

P. GUHATHAKURTA

UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064

J. HIBBARD

Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822

Y.-C. KIM

Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520

DANIEL M. AND E.REHNER SICILIANO

Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218

JOSHUA ROTH

Sky Publishing Corporation, 49 Bay State Road, Cambridge, MA 02138

PATRICK SEITZER

Department of Astronomy, University of Michigan, 830 Dennison Building, Ann Arbor, MI 48109

AND

T. B. WILLIAMS

Department of Physics and Astronomy, Rutgers University, P.O. Box 849, Piscataway, NJ 08855Received 1997 July 14 ; revised 1997 September 24

ABSTRACTWe present UBV RI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN

1991T in NGC 4527, based on photometry gathered in the course of the Cala� n/Tololo supernovaprogram. Both objects have well-sampled light curves starting several days before maximum light andspanning well through the exponential tail. These data supersede the preliminary photometry publishedby Leibundgut et al. and Phillips et al. The host galaxies for these supernovae have (or will have) accu-rate distances based on the Cepheid period-luminosity relationship. The photometric data in this paperprovide template curves for the study of the general population of Type Ia supernovae and accuratephotometric indices, needed for the Cepheid-supernova distance scale.Key words : galaxies : distances and redshifts È supernovae : individual (1990N, 1991T)

1 Current address : Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland, UK.2 Current address : Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721.3 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in

Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation.4 Current address : Department of Astronomy, University of Michigan, 830 Dennison Building, Ann Arbor, MI 48109.5 Current address : Kitt Peak National Observatory, National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85726.6 Hubble Fellow.7 Current address : NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771.8 Current address : Princeton University Observatory, Peyton Hall, Princeton, NJ 08544.

234

SNe 1990N AND 1991T 235

1. INTRODUCTION

It is now clear that Type Ia supernovae are not a homo-genous class of objects. One can see di†erences in spectralfeatures at speciÐc epochs (Bertola 1964 ; Branch 1987 ;

et al. & Wheeler et al.Phillips 1987 ; Harkness 1990 ; Nugentand in the overall morphology of1995 ; Filippenko 1997)

the light curves et al.(Phillips 1987 ; Phillips 1993 ; Suntze†which had long been suspected by earlier workers1996),

Ciatti, & Rosino(Barbon, 1973 ; Rust 1974 ; Pskovskii 1977 ;The modern data haveBranch 1981 ; Pskovskii 1984).

shown us, however, that the class of Type Ia supernovae canstill be used to provide accurate relative distances by apply-ing correction factors to the observed luminosity that are asimple function of the evolution of the light curve nearmaximum light et al.(Phillips 1993 ; Hamuy 1995 ; Riess,Press, & Kirshner The simple proof that these tech-1995).niques work is the reduction in the magnitude scatter of aHubble diagram for distant supernovae, where the scatterreduces from about 0.4 mag to 0.14 mag et al.(Hamuy

Press, & Kirshner1996b ; Riess, 1996).To use the very high accuracy of the zero point of the

distant supernova Hubble diagram to measure an accurateabsolute distance scale requires the direct measurements ofthe distances to a number of nearby galaxies that have hadwell-observed Type Ia supernovae (SNe). One of the mostaccurate methods to measure absolute distances to nearbygalaxies is the use of the Cepheid period-magnituderelationship calibrated relative to the Large MagellanicCloud & Freedman With an independent(Madore 1991).measurement of the distance to the LMC, the observedHubble diagram of distant supernovae will directly yield theHubble constant. The nearby calibrating galaxies musthave three rather trivial properties : the galaxy must beyoung enough to form classical Cepheids, the galaxy musthave hosted a reasonably ““ normal ÏÏ Type Ia supernova,and the supernova light curve must have been reasonablywell measured.

There are only a handful of such supernova host galaxieswithin the light grasp of the Hubble Space Telescope (HST ),where the limiting distance modulus for measuring Cepheidlight curves is about 32.0. Six supernovae, SNe 1895B,1937C, 1960F, 1972E, 1981B, and 1990N, have been cali-brated to date by Saha, Sandage, and collaborators (see

et al. for the most recent paper in this series).Saha 1996We have pointed out et al. that a few of(Hamuy 1996b)

the nearby supernovae are not ideal as calibrators. The lightcurves for SNe 1895B and 1960F are very poor, with ill-deÐned maxima. SN 1937C was well observed, but thetransformation from the 60 yearÈold Ðlms to modernphotometric bandpasses, while carefully calibrated (Pierce& Jacoby remains controversial in some circles1995),

rebuttal in & Pierce Both(Schaefer 1996 ; Jacoby 1996).1937C and 1972E clearly had ““ slower ÏÏ evolution nearmaximum light, which we have shown is indicative of intrin-sically brighter supernovae et al. however,(Hamuy 1996a) ;this latter point has been contested & Sandage(Tammann

et al. SN 1989B had very high1995 ; Sandage 1996).reddening [E(B[V )D 0.4 ; et al. Only SNeWells 1994].1981B and 1990N have evidently both uncontroversial lightcurves and Cepheid distances.

If we relax the requirement that the Cepheids must bemeasured in the same galaxy as the supernova and rely ongroup or cluster associations between galaxies, a number of

other calibrators become available. Cepheid distances toNGC 3368 (M96) et al. and NGC 3351(Tanvir 1995)

et al. have been measured with HST data.(Graham 1997)et al. associate the M96 Group with a largerSandage (1996)

Leo Group (also called the Leo I cloud), which includes thecompact M66 (Leo Triplet) Group. However, M66, whichhosted SN 1989B, is some 8¡ away from the Cepheid hostgalaxies, and one must worry that the whole Leo Group isat the same distance. HST observations to determine aCepheid distance to M66 are planned for HST Cycle 7 bySaha, Sandage, and collaborators. There have also been twowell-studied Type Ia supernovae in the Fornax Cluster : SN1980N (in NGC 1316) and SN 1992A (in NGC 1380).

et al. have measured a Cepheid distanceSilbermann (1996)to the peculiar spiral NGC 1365, thought to be a member ofFornax. Once again, the physical association of the super-nova host galaxy and the Cepheid host galaxy is a point ofsome controversy. Such ambiguities lead us to prefer TypeIa supernova absolute magnitude calibrations based on gal-axies that have both primary calibrators, such as Cepheidsand supernova.

SN 1990N was discovered signiÐcantly before maximumin the SBb(r) galaxy NGC 4639 by E. Thouvenot at theObservatory of the Coü te dÏAzur on 1990 June 22 (Maury

all dates referenced as UT). measured an1990 ; Pollas (1990)astrometric position for this supernova of (R.A., decl.,

B1950.0).equinox)\ (15h18m52s.92, [7¡11@43A.2, Kirshner& Leibundgut classiÐed it as a Type Ia supernova on(1990)the basis of a spectrum obtained on 1990 June 26.

et al. presented preliminary light curvesLeibundgut (1991)based on Cerro Tololo Inter-American Observatory(CTIO) CCD data (which are reanalyzed in the presentpaper). Spectral modeling has been published by etJe†eryal. et al. et al.(1992), Shigeyama (1992), Yamaoka (1992),and et al. Recently, et al. andFisher (1997). Sandage (1996)

et al. have obtained the distance to the parentSaha (1996)galaxy by measuring the periods and magnitudes of 20Cepheid variable stars with HST . This object is therefore akey template objects in establishing the value of basedH0on the Cepheid-supernova distance scale.

SN 1991T has been one of the most extensively studiedType Ia supernovae. It was discovered well beforemaximum in the Sb(s) II galaxy NGC 4527 by S. Knightand independent observers et al. on 1991(Waagen 1991)April 13. An astrometric position for this supernova by R.H. McNaught is given in the previous reference as (R.A.,decl., J2000.0).equinox)\ (12h31m36s.91, ]2¡56@28A.3,

Early optical spectral observations reported by LaFranca & Goldschmidt and(1991), Kirshner (1991), Phillips& Hamuy showed that SN 1991T was a peculiar(1991)Type Ia event, which motivated a number of theoreticalstudies to model the spectral evolution et al.(Je†ery 1992 ;

et al. et al.Ruiz-Lapuente 1992 ; Spyromilio 1992 ; Yamaokaet al. Danziger, & Turatto et1992 ; Mazzali, 1995 ; Meikleal. Optical photometry has been published by1996).

et al. et al. and et al.Phillips (1992), Ford (1993), Schmidtshowing that SN 1991T had a very slow rate of evol-(1994)

ution through maximum. Because of the excellent temporalcoverage of the light curve, this supernova has been used asa template example of a slow supernova et al.(Hamuy

et al. It is expected that the Saha and1996c ; Riess 1996).Sandage group will obtain a Cepheid distance to NGC 4527using data taken with HST in Cycle 7.

When accurate modern light curves for several nearby

236 LIRA ET AL. Vol. 115

supernovae became available some years ago, subtle di†er-ences between Type Ia supernova light curves becameapparent. CCD photometry showed that there is a realspread in the peak luminosity and that some of the objectsevolve through maximum light more slowly than others.

presented evidence that the rate of thePhillips (1993)decline after maximum is correlated with the luminosity atmaximum and that more luminous objects have a slowerdecline rates. et al. et al.Hamuy (1995), Riess (1995), Hamuyet al. and et al. have found that the(1996b), Riess (1996)scatter in the Hubble diagram of Type Ia supernovaedecreases signiÐcantly when corrections for the peakluminosityÈdecline rate relation are introduced. Tammann& Sandage argued that when samples are restricted(1995)to ““ normal ÏÏ objects (by eliminating events like SN 1991T)there is no need to correct for a peak luminosityÈdeclinerate e†ect. However, the Hamuy studies Ðnd that by ignor-ing this e†ect, the estimate of the Hubble constant can bebiased too low by up to 15%.

The intent of this paper is to present accurate light curvesof the two nearby supernovae, SN 1990N and SN 1991T,that are important calibrators of the distance scale. Wehave already used these light curves in our work on theHubble constant et al. In of this paper,(Hamuy 1996b). ° 2we present the observations and reduction of the opticalphotometric data obtained at CTIO. The light curves, aswell as color curves, for both supernovae are shown in A° 3.Ðnal discussion is found in ° 4.

2. OBSERVATIONS

The optical observations of SNe 1990N and 1991T wereobtained using the 0.9 m and the Blanco 4 m telescopes atCTIO. SN 1990N was observed from 1990 June to 1992March, and SN 1991T was observed from 1991 April to1992 June. The observations were made using Texas Instru-ments and Tektronix CCDs (except for the night of 1990March 19, when a Thomson detector was used) and facility

Ðlters in the Johnson-Kron-Cousins photo-UBV (RI)KCmetric system (Johnson 1963 ; Kron 1953 ; Cousins 1976).The observation logs are given in Tables and The detec-1 2.tor name, listed in the Ðnal column of these two tables,combines the manufacturer name and a running numberassigned by the CTIO CCD lab. We have assumed thateach di†erent CCD listed in this table (along with the Ðlterset) has a unique set of color terms that must be derivedfrom observations.

We made observations of the supernovae under variedphotometric conditions, including very nonphotometricweather with cloud extinction up to a few magnitudes. It iswell established et al.(Serkowski 1970 ; Walker 1971 ; Olsen

that clouds are quite gray, which allows us to use1983)local standards (in the same CCD frame as the supernova)and averaged color terms for a speciÐc CCD measured onphotometric nights.

For accurate photometry on nonphotometric nights, it isnecessary to deÐne a precise local photometric sequence ofstars. We measured photometric sequences on 13 photo-metric nights in the CCD Ðeld around NGC 4639 and NGC4527 referenced to Landolt and Graham standards stars

Extinction(Landolt 1972 ; Graham 1982 ; Landolt 1992).coefficients, color terms, and zero points for the transform-ations to the standard system were derived forUBV (RI)KCeach night following the method described by Fitz-Harris,gerald, & Reed Typical values of the extinction coef-(1981).

Ðcients were andkU

\ 0.50, kB\ 0.32, k

V\ 0.20, k

R\ 0.14,

in units of mag We measuredkI\ 0.08, airmass~1.10

sequences for a total of 15 stars for SN 1990NUBV (RI)KCand nine stars for SN 1991T using digital aperture photo-metry with an aperture diameter of 14A. The photometricsequences are identiÐed in Figures and and the photo-1 2,metry is given in Tables and In these tables we list the3 4.number of observing nights (n) and the total number ofobservations (m).

Star 2 in our local sequence around SN 1991T is also asequence star (number 2) listed by et al. TheFord (1993).magnitude di†erences for this star in the sense of this studyminus Ford et al.Ïs are *(V RI) \ (0.00, [0.02, [0.03). Forthe three SN 1991T local standards in common with

et al. we Ðnd the mean di†erences in theSchmidt (1994),sense of this study minus that of et al. areSchmidt (1994)*(BV RI)\ ([0.03^ 0.01, [0.04^ 0.01, [0.02^ 0.02,]0.12^ 0.08), where the errors quoted are the mean errors.These mean di†erences are consistent with the photometricerrors quoted in et al. which dominate theSchmidt (1994),comparison of the two magnitude systems. As a result of thelarger number of measurements on independent photo-metric nights, we are conÐdent that sequences given inTables and are the most accurate available.3 4

To determine the supernova magnitudes, we subtractedlate-time images of the parent galaxies at the location of thesupernovae using the technique described by et al.Hamuy

For these subtractions, deep ““master images ÏÏ of(1994).NGC 4639 and NGC 4527 were obtained at the beginningof 1994, which corresponds to 1300 and 1010 days aftermaximum light for SNe 1990N and 1991T, respectively. Asimple extrapolation of the late-time decline rates (see thec-parameter in below) to these dates yields B magni-Table 8tudes of D33 (SN 1990N) and D28 (SN 1991T). However,there are two factors that could make the late-time magni-tudes in the master images signiÐcantly brighter than thisextrapolation.

The Ðrst factor is the presence of minor radioactivenuclides and the efficiency of positron energy depositionfrom the radioactive decays. A Type Ia explosion is predict-ed to synthesize about 0.5 of 56Ni and smaller amountsM

_of 56Co, 57Co, 44Ti, and 22Na (see model W7 in Nomoto,Thielemann, & Yokoi Pinot, & Hartmann1994). Woosley,

provide energy deposition rates for these nuclides. A(1989)complication arises when predicting the late-time lightcurve after day 500 in how to handle the energy depositionfrom the positron production The positrons(Arnett 1979).can add energy into the supernova nebula from both theirkinetic energies and annihilations. The efficiency of thisprocess is poorly understood in the low-density environ-ment of the supernova nebula at late time. If we make therather extreme assumption of complete kinetic energy depo-sition and positron annihilation into gamma rays, we canuse the deposition rates given by et al. andWoosley (1989)the model W7 abundances by et al. toNomoto (1994)predict upper limits to the supernova luminosity at latetime. The e†ect of full energy deposition from positrons andthe existence of long-lived radioactive nuclides such as 44Tiand 22Na tends to Ñatten out the light curve past day 1000.

10 These extinction values are higher than normal because of the e†ectsof the Mount Pinatubo eruption that occurred on JD 2,448,422 (see

& GochermannGrothues 1992).

No. 1, 1998 SNe 1990N AND 1991T 237

TABLE 1

OBSERVATION LOG FOR SN 1990N

Date (UT) Telescope Observers Filters Detector

1990 Jun 29 . . . . . . 4.0 m M. N. UBV RI TI21990 Jul 3 . . . . . . . . . 0.91 m D. M. R., E. S. BV R TEK41990 Jul 4 . . . . . . . . . 0.91 m D. M. R., E. S. BV R TEK41990 Jul 4 . . . . . . . . . 0.91 m N. B. S. BV RI TEK41990 Jul 12 . . . . . . . 0.91 m L. A. W. UBV RI TEK41990 Jul 19 . . . . . . . 0.91 m N. D. T., R. M. R. UBV RI TI21990 Jul 20 . . . . . . . 0.91 m N. D. T., R. M. R. UBV RI TI21990 Jul 21 . . . . . . . 0.91 m N. D. T., R. M. R. UBV RI TI21990 Jul 22 . . . . . . . 0.91 m N. D. T., R. M. R. UBV RI TI21990 Jul 25 . . . . . . . 0.91 m T. B. W. BV RI TEK41990 Jul 26 . . . . . . . 0.91 m T. B. W. BV RI TEK41990 Aug 1 . . . . . . . 0.91 m N. B. S. UBV RI TI31990 Aug 21 . . . . . . 0.91 m J. H. E. BV RI TI31990 Dec 28 . . . . . . 0.91 m F. K. B. BV RI TI31991 Jan 16 . . . . . . . 0.91 m L. G. BV RI TI31991 Feb 5 . . . . . . . . 0.91 m M. N. BV RI TI21991 Feb 12 . . . . . . 4.0 m M. N. UBV RI TEK11991 Feb 20 . . . . . . 0.91 m M. N. BV RI TEK41991 Mar 7 . . . . . . . 0.91 m N. B. S. BV RI TI31991 Mar 19 . . . . . . 0.91 m J. R. V I Thomson1991 Mar 20 . . . . . . 0.91 m N. B. S. BV RI TI31991 Apr 1 . . . . . . . . 0.91 m A. P. S. C. UBV RI TEK11991 Apr 7 . . . . . . . . 0.91 m P. U. BV RI TEK21991 Apr 9 . . . . . . . . 4.0 m M. N. BV RI TEK11991 Jun 7 . . . . . . . . 4.0 m M. N. BV RI TI21991 Jun 13 . . . . . . 0.91 m M. N. BV R TEK41991 Jul 23 . . . . . . . 0.91 m A. D. BV TEK21992 Mar 6 . . . . . . . 4.0 m N. B. S., M. N. V R TEK2

Using a column depth of 400 g cm~2 at t \ 106 s as sug-gested by et al. for a Type Ia supernova, weWoosley (1989)predict V magnitudes of 29.6 and 25.4 for SNe 1990N and1991T, respectively, at the epoch of the master image. Forno positron energy deposition, the predicted magnitudesare about 3 mag fainter. In either case, the predicted magni-tudes in the master images are so faint as to have no e†ecton the measured photometry. However, if there is signiÐ-cant overproduction of 57Co or 44Ti relative to model W7,the late-time magnitudes could be much brighter and a†ectthe magnitudes measured by image subtraction.

The second factor that could a†ect the magnitudes mea-sured from the subtracted images is the presence of a lightecho. In the late-time images of SN 1991T, the location ofthe supernova has been found to be contaminated by a faintecho of SN 1991T at maximum light et al.(Schmidt 1994).We will return to this minor complication in ° 3.

We measured di†erential photometry of the supernovaeon each CCD frame, using aperture photometry when thesupernova was bright and using the point-spread function(PSF) Ðtting program DAOPHOT when the(Stetson 1987)supernova was faint. Averaged color terms chosen to matchthe CCD/Ðlter setup of instruments for each observingnight were adopted from a database of coefficients at CTIO.For SN 1991T near maximum, our CCD exposures werevery short and the local standards were poorly exposed. Inthis case we used the sharp core of NGC 4527 as a BV““ standard ÏÏ for the nights of 1991 April 26, 28, 29, and 30and May 1. An aperture radius of was chosen to maxi-2A.7mize the signal-to-noise ratio for the photometry of thecore. The core photometry is listed in Table 4.

Besides the error given by the Poisson statistics of thenumber of counts in the supernova aperture or PSF, pphot,there are other error sources, such as those due to the trans-formation of the instrumental magnitudes into the standard

system and CCD Ñat-Ðelding. To get a sense of the magni-tude of these errors, we selected many frames with a suffi-cient number of bright stars (with negligible For eachpphot).frame, we calculated the standard deviation of the di†erencebetween a given measurement and the standard magnitudeslisted in Tables and This standard deviation is an3 4. (prms)empirical estimate of the average error in a single obser-vation of a stellar object in any CCD frame when referencedto the local photometric sequence. The measured standarddeviations were 0.026, 0.017, 0.017, 0.015, and 0.017prmsmag in UBV RI, respectively. The value of derived forprmsboth supernovae in each Ðlter agreed within 0.002 mag.

The Ðnal error in the individual magnitudes of the super-novae was calculated as the quadratic sum of the empiricalerror in a single observation, and the photon statisticalprms,error, The error was the dominant component ofpphot. prmsthe errors in the early part of the photometry, while pphotbecame more important when the supernova dimmed.

3. RESULTS

3.1. L ight CurvesWe present photometry for SNe 1990N andUBV (RI)KC1991T in Tables and and plot the data in Figures and5 6 3To Ðnd the time and magnitude of maximum light for4.

both supernovae, we Ðtted the data around the peak with athird-order polynomial. For SN 1990N the Ðrst observationwas acquired 11 days before and the last observationBmax,was made 607 days after in just the V band. The BBmaxmaximum was reached on JD 2,448,082.7^ 0.5, with

and a B[V color of 0.03 mag. For SNBmax\ 12.76^ 0.031991T the data begin 12 days before and end 401 daysBmaxafter maximum. We derive at JD 2,448,Bmax\ 11.70^ 0.02375.7^ 0.5 with a B[V color of 0.17. In we sum-Table 7,marize the maxima of the light curves in the di†erent bands

238 LIRA ET AL. Vol. 115

TABLE 2

OBSERVATION LOG FOR SN 1991T

Date (UT) Telescope Observers Filters Detector

1991 Apr 17 . . . . . . 0.91 m J. H., P. G. BV R TEK41991 Apr 18 . . . . . . 0.91 m J. H., P. G. BV R TEK41991 Apr 19 . . . . . . 0.91 m R. A. S. UBV RI TI31991 Apr 20 . . . . . . 0.91 m J. A. B. BV R TI31991 Apr 26 . . . . . . 0.91 m R. A. S. BV RI TEK11991 Apr 27 . . . . . . 0.91 m R. A. S. UBV I TEK11991 Apr 28 . . . . . . 0.91 m L. A. W. UBV RI TI31991 Apr 29 . . . . . . 0.91 m L. A. W. UBV RI TI31991 Apr 30 . . . . . . 0.91 m L. A. W. UBV RI TI31991 May 1 . . . . . . . 0.91 m L. A. W. UBV RI TEK41991 May 4 . . . . . . . 0.91 m G. M. W. BV RI TEK41991 May 5 . . . . . . . 0.91 m G. M. W. BV RI TEK41991 May 6 . . . . . . . 0.91 m G. M. W. BV RI TEK41991 May 7 . . . . . . . 0.91 m M. N. UBV RI TEK41991 May 9 . . . . . . . 4.0 m M. N. BV RI TEK11991 May 10 . . . . . . 0.91 m A. R. W. BV RI TI31991 May 12 . . . . . . 0.91 m N. D. T. UBV RI TEK41991 May 14 . . . . . . 0.91 m L. A. W. UBV RI TI31991 May 17 . . . . . . 0.91 m L. G. UBV RI TI31991 May 19 . . . . . . 0.91 m A. L. BV TI31991 May 29 . . . . . . 0.91 m G. M. W. BV RI TEK21991 Jun 7 . . . . . . . . 4.0 m M. N. UBV RI TI21991 Jun 13 . . . . . . . 0.91 m M. N. UBV RI TEK41991 Jun 24 . . . . . . . 0.91 m R. A. S. UBV RI TEK21991 Jul 1 . . . . . . . . . 0.91 m P. S. BV I TEK11991 Jul 2 . . . . . . . . . 0.91 m R. A. S. BV RI TI31991 Jul 3 . . . . . . . . . 0.91 m T. B. W. BV RI TI31991 Jul 5 . . . . . . . . . 0.91 m M. N. UBV RI TI31991 Jul 8 . . . . . . . . . 0.91 m E. S. BV TEK21991 Jul 22 . . . . . . . . 0.91 m A. D. BV RI TEK21992 Jan 7 . . . . . . . . 0.91 m R. A. S. BV I TEK21992 Jan 8 . . . . . . . . 0.91 m R. A. S. BV I TEK21992 Feb 25 . . . . . . 0.91 m E. S. V I TEK11992 Mar 6 . . . . . . . 4.0 m N. B. S., M. N. BV RI TEK11992 Mar 10 . . . . . . 0.91 m M. H. BV RI TEK41992 Apr 8 . . . . . . . . 0.91 m Y.-C. K. BV I TEK21992 Apr 19 . . . . . . 0.91 m R. A. S. BV TEK11992 Jun 4 . . . . . . . . 0.91 m L. A. W. UBV RI TEK4

for both supernovae. We Ðnd that the B maximum occursbefore the V maximum; in particular, the time di†erencebetween the B and V maxima is 1.5^ 0.7 days for SN1990N and 2.6 ^ 0.7 days for SN 1991T, in agreement withthe result of who found a di†erence ofLeibundgut (1988),2.5^ 0.5 days.

Comparisons of our BV photometry with previous datapublished for SNe 1990N and 1991T are shown in Figures 5and The agreement between the photometry presented in6.this paper and the results published by et al.Leibundgut

for SN 1990N and et al. for SN 1991T(1991) Phillips (1992)is not surprising, since they are based on a subsample of the

TABLE 3

PHOTOMETRIC SEQUENCE FOR SN 1990N

V (m.e.) B[V (m.e.) V [R (m.e.) R[I (m.e.) U[B (m.e.)Stara n m (mag) (mag) (mag) (mag) (mag)

1 . . . . . . . 5 9 15.728 (005) 1.076 (009) 0.646 (006) 0.512 (011) 1.002 (026)2 . . . . . . . 5 8 17.810 (003) 0.659 (007) 0.414 (004) 0.457 (009) 0.013 (012)3 . . . . . . . 7 13 18.514 (004) 0.702 (005) 0.453 (005) 0.457 (006) 0.013 (021)4 . . . . . . . 6 12 17.118 (004) 0.623 (006) 0.379 (005) 0.362 (006) 0.074 (009)5 . . . . . . . 7 13 18.846 (005) 0.730 (007) 0.435 (006) 0.408 (009) 0.280 (033)6 . . . . . . . 7 13 18.622 (005) 0.519 (008) 0.333 (006) 0.362 (009) [0.086 (024)7 . . . . . . . 6 12 17.707 (003) 0.735 (006) 0.414 (004) 0.402 (005) 0.068 (018)8 . . . . . . . 3 6 17.626 (003) 1.468 (006) 0.940 (004) 0.933 (005) 1.256 (092)9 . . . . . . . 7 12 19.543 (005) 0.479 (010) 0.377 (006) 0.466 (010) [0.129 (021)10 . . . . . . 7 12 19.573 (010) 0.863 (026) 0.539 (013) 0.705 (014) [0.269 (074)11 . . . . . . 5 9 16.580 (004) 0.615 (007) 0.385 (006) 0.351 (006) 0.040 (008)12 . . . . . . 7 13 19.140 (007) 1.484 (023) 1.104 (013) 1.509 (014) 0.601 (066)13 . . . . . . 5 9 18.467 (010) 1.285 (021) 0.860 (011) 0.871 (006) 0.941 (062)14 . . . . . . 4 9 16.602 (005) 1.455 (007) 0.955 (009) 0.996 (009) 1.072 (031)15 . . . . . . 3 6 13.392 (001) 0.607 (002) 0.356 (002) 0.341 (003) 0.031 (200)

a See Fig. 1.

No. 1, 1998 SNe 1990N AND 1991T 239

FIG. 1.ÈSN 1990N in NGC 4639. The local standards listed in are marked. This R image was taken at the CTIO 0.9 m telescope on 1994 MarchTable 313. The Ðeld is on a side. Star 14 does not appear on this chart. It is located at ]13¡17@52A, J2000.0) on the Digitized Sky Survey.6A.8 (12h42m47s.0,

same optical data analyzed here. However, a small system-atic di†erence between the di†erent data sets is clear. Ourphotometry near maximum is generally dimmer, althoughthe discrepancy is less than 0.1 mag for SN 1990N and evenless for SN 1991T. The preliminary photometric results ofthese earlier papers, which were based on only a single night

for the photometric calibration, should be ignored in prefer-ence of the photometric data given in Tables and5 6.

For SN 1991T there is independent photometrypublished by et al. which we plot in IfFord (1993), Figure 6.we interpolate our data to the dates of the et al.Ford (1993)data using a spline Ðt, we Ðnd the following di†erences in

TABLE 4

PHOTOMETRIC SEQUENCE FOR SN 1991T

V (m.e.) B[V (m.e.) V [R (m.e.) R[I (m.e.) U[B (m.e.)Stara n m (mag) (mag) (mag) (mag) (mag)

1 . . . . . . . 6 10 15.172 (009) 0.908 (010) 0.523 (011) 0.483 (007) 0.647 (022)2b . . . . . . 7 11 15.289 (009) 1.468 (011) 1.006 (012) 1.057 (011) 1.234 (033)3 . . . . . . . 6 8 14.367 (014) 1.338 (026) 0.694 (014) 0.657 (006) 1.257 (053)4 . . . . . . . 6 10 15.164 (006) 0.768 (006) 0.445 (008) 0.393 (009) 0.709 (019)5 . . . . . . . 3 6 14.900 (007) 0.757 (007) 0.442 (009) 0.408 (006) 0.380 (005)6 . . . . . . . 3 6 16.744 (005) 1.548 (009) 0.994 (009) 1.092 (014) 1.068 (008)7c . . . . . . . 8 12 16.733 (003) 1.173 (009) 0.721 (006) 0.630 (010) 1.097 (011)8c . . . . . . . 7 11 16.304 (003) 1.480 (006) 0.959 (007) 0.979 (006) 1.075 (042)9c . . . . . . . 8 11 18.488 (010) 0.441 (011) 0.298 (012) 0.302 (017) [0.241 (014)Gd . . . . . . 4 7 14.297 (007) 1.290 (007) 0.837 (010) 0.885 (009) 0.241 (105)

a See Fig. 2.b Star 2 is also star 2 in et al.Ford 1993.c Stars 7, 8, and 9 are stars 2, 1, and 3 of et al. respectively.Schmidt 1994,d G corresponds to the photometry of the NGC 4527 nucleus.

FIG. 2.ÈSN 1991T in NGC 4527. The local standards listed in are marked. This R image was taken at the CTIO 0.9 m telescope on 1994 MarchTable 414. The Ðeld is on a side.6A.8

FIG. 3.ÈUBV RI light curves of SN 1990N FIG. 4.ÈUBV RI light curves of SN 1991T

SNe 1990N AND 1991T 241

TABLE 5

SN 1990N PHOTOMETRY

U (m.e.) B (m.e.) V (m.e.) R (m.e.) I (m.e.)JD[ 2,440,000 (mag) (mag) (mag) (mag) (mag)

8,071.50 . . . . . . . . 13.645 (026) 13.894 (017) 13.837 (017) 13.740 (013) 13.807 (015)8,071.50 . . . . . . . . 13.625 (026) 13.893 (017) 13.834 (017) 13.721 (013) 13.815 (016)8,071.50 . . . . . . . . 13.671 (026) 13.888 (017) 13.837 (017) 13.742 (013) 13.817 (016)8,072.47 . . . . . . . . . . . 13.638 (017) 13.628 (017) 13.529 (013) . . .8,072.48 . . . . . . . . . . . 13.636 (017) 13.624 (017) 13.525 (013) . . .8,075.53 . . . . . . . . . . . 13.131 (017) 13.141 (017) 13.046 (013) . . .8,075.53 . . . . . . . . . . . 13.132 (017) 13.142 (017) 13.046 (013) . . .8,076.56 . . . . . . . . . . . 13.053 (017) 13.053 (017) 12.976 (013) 13.070 (015)8,076.56 . . . . . . . . . . . 13.054 (017) 13.048 (017) 12.972 (013) 13.064 (015)8,084.49 . . . . . . . . 12.438 (026) 12.790 (017) 12.732 (017) 12.731 (013) 13.026 (015)8,084.49 . . . . . . . . 12.446 (026) 12.784 (017) 12.730 (017) 12.727 (013) 13.027 (015)8,091.53 . . . . . . . . 13.018 (026) 13.133 (017) 12.874 (017) 12.914 (013) 13.289 (016)8,091.53 . . . . . . . . 13.032 (026) . . . . . . . . . . . .8,091.53 . . . . . . . . 13.014 (026) . . . . . . . . . . . .8,093.51 . . . . . . . . 13.257 (026) 13.318 (017) 12.971 (017) 13.065 (013) 13.377 (016)8,093.51 . . . . . . . . . . . . . . . . . 13.079 (013) . . .8,094.51 . . . . . . . . 13.386 (026) 13.443 (017) 13.057 (017) 13.202 (013) 13.478 (016)8,095.49 . . . . . . . . 13.500 (026) 13.502 (017) 13.092 (017) 13.206 (013) 13.460 (016)8,097.51 . . . . . . . . . . . 13.774 (017) 13.236 (017) 13.328 (013) 13.550 (015)8,098.52 . . . . . . . . . . . 13.885 (017) 13.264 (017) 13.341 (013) 13.531 (015)8,104.48 . . . . . . . . 14.712 (026) 14.522 (017) 13.614 (017) 13.464 (013) 13.428 (015)8,104.50 . . . . . . . . . . . 14.520 (017) 13.618 (017) 13.468 (013) 13.424 (015)8,124.47 . . . . . . . . . . . 15.753 (022) 14.608 (018) 14.252 (021) 14.012 (018)8,124.47 . . . . . . . . . . . . . . . . . 14.271 (018) . . .8,253.83 . . . . . . . . . . . 17.492 (019) 17.539 (018) 18.145 (019) 18.053 (045)8,272.82 . . . . . . . . . . . 17.958 (018) 17.796 (018) 18.317 (020) 18.227 (041)8,292.76 . . . . . . . . . . . 18.049 (043) 17.899 (044) 18.455 (086) 18.787 (218)8,299.87 . . . . . . . . 19.588 (059) 18.118 (018) 18.242 (018) 18.765 (017) 18.682 (034)8,299.87 . . . . . . . . . . . . . . . . . 18.750 (027) 18.610 (040)8,307.80 . . . . . . . . . . . 18.299 (023) 18.286 (023) 18.989 (047) 18.824 (046)8,307.80 . . . . . . . . . . . . . . 18.394 (025) 18.916 (021) 18.782 (043)8,322.83 . . . . . . . . . . . 18.554 (027) 18.587 (029) 19.160 (047) 18.961 (050)8,334.76 . . . . . . . . . . . . . . 18.756 (027) . . . 19.185 (088)8,335.74 . . . . . . . . . . . 18.842 (021) 18.767 (023) 19.531 (038) 18.947 (061)8,347.70 . . . . . . . . . . . 18.771 (076)a 18.941 (048)a 19.550 (057)a 19.378 (131)a8,347.74 . . . . . . . . . . . 18.829 (092)a 18.900 (046)a 19.635 (073)a 19.318 (125)a8,353.69 . . . . . . . . . . . 18.932 (028) 18.981 (026) 19.764 (041) 19.291 (066)8,355.69 . . . . . . . . . . . 18.864 (018) 19.053 (019) 19.711 (018) 19.235 (025)8,414.61 . . . . . . . . . . . 19.960 (035)a 20.114 (038)a 20.853 (042)a 20.107 (059)a8,414.61 . . . . . . . . . . . . . . . . . . . . 20.048 (070)a8,420.54 . . . . . . . . . . . 19.888 (044) 19.931 (051) 20.527 (121) . . .8,459.51 . . . . . . . . . . . 20.541 (149)a 20.904 (231)a . . . . . .8,687.82 . . . . . . . . . . . . . . 23.198 (114)a . . . . . .

a PSF photometry.

the sense of this work minus Ford et al. : V , 0.08 ^ 0.01 ; R,0.04^ 0.02 ; and I, 0.02^ 0.02. The quoted errors are theerrors in the mean based on the interpolation to the 12dates in the Ford et al. study. A similar systematic o†set inthe V magnitude was noted by Ford et al. with respect tothe et al. reductions of the SN 1991T data.Phillips (1992)These mean di†erences between careful photometric studiesindicate the level in systematic errors that can be encoun-tered even in bright supernova photometry.

It is now well established that there is not a unique lightcurve for all Type Ia supernovae. As was suggested by

and supernovae can be dis-Pskovskii (1977) Branch (1981),criminated by the rate of decline after maximum. Pskovskii

deÐned the parameter b as the characteristic(1977, 1984)decline rate during the fast-decline phase of the B supernovalight curve and the parameter c as the rate during the slow-decline phase (see et al. for an unambiguousPhillips 1987description of these parameters). introducedPhillips (1993)the parameter deÐned as the decline in magnitude*m15,

during the 15 days after B maximum. The evidence fromnearby supernovae et al. and(Phillips 1993 ; Hamuy 1996c)the scatter in the observed Hubble diagram et al.(Maza

Hamuy et al. clearly show that the1994 ; 1995, 1996b)brighter supernovae decline more slowly (small *m15).In we list the evolutionary parameters of the BTable 8,light curves for our two supernovae. We also list the valuesfor the template B curve. This multicolorLeibundgut (1988)template, which was formed from a large number of super-nova light curves, provides a useful Ðducial light curve andcan be considered a ““ typical ÏÏ light curve that can be com-pared with other observations. We calculated the Pskovskiib- and c-parameters for the B curves of SNe 1990N and1991T by using a linear least-squares Ðt, with the dataweighted using the photometric errors as quoted in Tables 5and The range of days used in the Ðtting for each super-6.nova are indicated in Table 8.

shows that both b(B) and are much smallerTable 8 *m15for SN 1991T than the values for the Leibundgut template.

242 LIRA ET AL. Vol. 115

TABLE 6

SN 1991T PHOTOMETRY

U (m.e.) B (m.e.) V (m.e.) R (m.e.) I (m.e.)JD[ 2,440,000 (mag) (mag) (mag) (mag) (mag)

8,363.77 . . . . . . . . . . . 12.712 (016) 12.618 (016) 12.526 (015) . . .8,364.76 . . . . . . . . . . . 12.449 (016) 12.362 (016) 12.299 (015) . . .8,365.66 . . . . . . . . 11.610 (026) 12.362 (016) 12.256 (016) 12.134 (015) 12.215 (017)8,365.65 . . . . . . . . . . . . . . . . . 12.159 (015) . . .8,366.67 . . . . . . . . . . . 12.152 (016) 12.061 (016) 11.971 (015) . . .8,372.70 . . . . . . . . . . . 11.748 (016)b . . . 11.533 (015) 11.693 (017)8,374.64 . . . . . . . . 11.282 (026)a 11.719 (016)b 11.573 (016)b 11.511 (015) 11.690 (017)8,374.64 . . . . . . . . 11.304 (026)a 11.707 (016)b 11.560 (016)b 11.519 (015) 11.694 (017)8,375.65 . . . . . . . . 11.370 (026)a 11.715 (016)b 11.568 (016)b 11.529 (015) 11.689 (017)8,375.65 . . . . . . . . 11.270 (026)a 11.720 (016)b 11.537 (016)b 11.509 (015) 11.676 (017)8,376.69 . . . . . . . . 11.279 (026)a 11.696 (016)b 11.542 (016)b 11.474 (015) . . .8,376.69 . . . . . . . . 11.233 (026)a 11.741 (016)b 11.527 (016)b 11.475 (015) . . .8,376.69 . . . . . . . . . . . . . . . . . . . . 11.690 (017)8,377.62 . . . . . . . . 11.329 (026)a 11.694 (016)b 11.511 (016)b 11.435 (015) 11.664 (017)8,377.62 . . . . . . . . . . . 11.711 (016)b 11.482 (016)b 11.435 (015) 11.665 (017)8,380.70 . . . . . . . . . . . 11.832 (016) 11.525 (016) 11.466 (015) 11.788 (017)8,380.69 . . . . . . . . . . . . . . 11.529 (016) 11.479 (015) 11.805 (018)8,381.60 . . . . . . . . . . . 11.836 (016) 11.535 (016) 11.518 (015) 11.847 (017)8,381.60 . . . . . . . . . . . 11.857 (016) . . . 11.501 (015) 11.846 (017)8,382.67 . . . . . . . . . . . 11.912 (016) 11.577 (016) 11.565 (015) 11.915 (017)8,382.68 . . . . . . . . . . . 11.919 (016) 11.575 (016) 11.552 (015) 11.917 (017)8,383.58 . . . . . . . . 11.652 (026) 11.959 (016) 11.605 (016) 11.589 (015) 11.966 (017)8,383.58 . . . . . . . . . . . 11.971 (016) 11.583 (016) 11.585 (015) 11.967 (017)8,385.63 . . . . . . . . . . . 12.137 (016) 11.643 (016) 11.730 (015) 12.051 (017)8,385.62 . . . . . . . . . . . . . . . . . 11.729 (015) 12.049 (017)8,386.55 . . . . . . . . . . . 12.248 (016) 11.764 (016) 11.811 (015) 12.087 (017)8,386.55 . . . . . . . . . . . 12.208 (016) . . . . . . . . .8,388.57 . . . . . . . . 12.381 (026) 12.408 (016) 11.873 (016) 11.917 (015) . . .8,390.50 . . . . . . . . 12.750 (026) 12.603 (016) 11.974 (016) 11.982 (015) 12.076 (017)8,390.50 . . . . . . . . 12.778 (026) 12.600 (016) 11.971 (016) 11.976 (015) 12.062 (017)8,393.62 . . . . . . . . 13.222 (026) 12.965 (016) 12.150 (016) 12.029 (015) 12.022 (017)8,395.55 . . . . . . . . . . . 13.131 (016) 12.223 (016) . . . . . .8,395.56 . . . . . . . . . . . 13.149 (016) 12.247 (016) . . . . . .8,405.59 . . . . . . . . . . . 14.071 (016) 12.731 (016) 12.378 (015) 12.098 (017)8,405.59 . . . . . . . . . . . 14.071 (017) 12.735 (016) 12.379 (015) 12.111 (017)8,414.62 . . . . . . . . 14.610 (026) 14.302 (016) 13.123 (016) 12.820 (015) 12.421 (017)8,414.62 . . . . . . . . . . . 14.307 (016) . . . . . . . . .8,420.47 . . . . . . . . 14.862 (026) 14.556 (016) 13.423 (016) 13.102 (015) 12.819 (017)8,431.49 . . . . . . . . 15.032 (030) 14.770 (017) 13.744 (016) 13.527 (015) 13.386 (017)8,431.49 . . . . . . . . 15.070 (030) 14.746 (017) 13.754 (016) . . . . . .8,438.50 . . . . . . . . . . . 14.863 (016) 13.936 (016) . . . 13.712 (017)8,439.47 . . . . . . . . . . . 14.931 (016) 13.986 (016) 13.830 (015) 13.738 (017)8,439.47 . . . . . . . . . . . 14.919 (016) 14.018 (016) 13.824 (015) 13.749 (017)8,440.46 . . . . . . . . . . . 14.897 (016) 14.059 (016) 13.859 (001) 13.821 (017)8,440.46 . . . . . . . . . . . 14.917 (016) 14.058 (016) 13.872 (015) 13.790 (017)8,442.48 . . . . . . . . 15.265 (026) 14.888 (016) 14.045 (016) 13.888 (015) 13.778 (017)8,445.46 . . . . . . . . . . . . . . 14.139 (016) . . . . . .8,445.47 . . . . . . . . . . . 14.934 (017) 14.143 (016) . . . . . .8,459.49 . . . . . . . . . . . 15.115 (016) 14.476 (016) 14.407 (015) 14.538 (017)8,628.83 . . . . . . . . . . . 17.496 (019) 17.484 (018) . . . 17.939 (040)8,629.83 . . . . . . . . . . . 17.513 (018) 17.475 (018) . . . 17.951 (046)8,677.78 . . . . . . . . . . . . . . 18.116 (022) . . . 18.367 (045)8,687.85 . . . . . . . . . . . . . . 18.343 (017) 19.086 (022) 18.489 (026)8,691.78 . . . . . . . . . . . 18.445 (018)a 18.357 (019)a 19.080 (025)a 18.489 (031)a8,720.77 . . . . . . . . . . . 18.834 (023)a 18.765 (022)a . . . 18.952 (034)a8,731.63 . . . . . . . . . . . 18.916 (078)a 18.947 (047)a . . . . . .8,731.62 . . . . . . . . . . . . . . 19.010 (052)a . . . . . .8,776.53 . . . . . . . . 20.346 (250)a 19.654 (078)a 19.662 (057)a 20.271 (102)a 19.374 (119)a8,776.54 . . . . . . . . . . . 19.773 (097)a 19.684 (063)a 20.162 (097)a 19.359 (113)a

a PSF photometry.b Photometry calibrated using the core of NGC 4527.

has shown that this ““ slow ÏÏ supernova wasPhillips (1993)intrinsically very bright. In fact, SN 1991T is one of theslowest supernovae ever found and has been used as a rep-resentative template for slow events (Hamuy et al. 1995,

SN 1990N, on the other hand, is quite similar to the1996c).

Leibundgut template and is therefore similar to the typicalType Ia event.

In Figures and we plot the Ðrst 120 days of BV7 8,photometry for the two supernovae along with the Lei-bundgut templates. The BV templates have been shifted to

No. 1, 1998 SNe 1990N AND 1991T 243

TABLE 7

PEAK PHOTOMETRIC MAGNITUDES

Filter Magnitude JD[ 2,440,000

SN 1990N:U . . . . . . 12.38^ 0.02 8,081.8^ 1.0B . . . . . . 12.76^ 0.03 8,082.7^ 0.5V . . . . . . 12.70^ 0.02 8,084.2^ 0.5R . . . . . . 12.69^ 0.02 8,082.9^ 0.5I . . . . . . . 12.94^ 0.02 8,080.7^ 1.0

SN 1991T:U . . . . . . 11.26^ 0.02 8,374.0^ 1.0B . . . . . . 11.70^ 0.02 8,375.7^ 0.5V . . . . . . 11.51^ 0.02 8,378.3^ 0.5R . . . . . . 11.46^ 0.02 8,377.1^ 0.5I . . . . . . . 11.67^ 0.02 8,375.3^ 1.0

match the epoch of B maxima and the peak magnitudesgiven in (with the appropriate time delay between BTable 7and V maximum given above). The results given in thepreceding paragraph can now be clearly seen in theseÐgures. SN 1990N follows the template closely while SN1991T declines from maximum light more slowly. SN 1991Talso begins its exponential decline signiÐcantly earlier andremains at higher relative brightness when compared withthe template. Visually, the ““ knee ÏÏ in the light curve around30 days past maximum occurs earlier in this supernova. SN1991T also rises to maximum light more slowly than SN1990N.

FIG. 5.ÈComparison of the B and V photometry of SN 1990N given inthis paper (circles) with the preliminary photometric reductions of theCTIO data by et al. (triangles).Leibundgut (1991)

FIG. 6.ÈComparison of our B and V light curves of SN 1991T (circles)with the preliminary photometric reductions of the CTIO data publishedby et al. (triangles). The V magnitudes from the Van VleckPhillips (1992)Observatory study by et al. are plotted as squares.Ford (1993)

In we plot the late-time photometry of SNFigure 9,1991T from this study and et al. along withSchmidt (1994)the predicted trend of the late-time evolution based on thec(B) and c(V ) Ðts to our data. The fact that the light curvepast day 400 levels o† has been shown by et al.Schmidt

to be due to a light echo with (B, V ) D (21.3, 21.4).(1994)

FIG. 7.ÈBV light curves of SN 1990N compared with the SN Ia tem-plate curves determined by The abscissa is plotted inLeibundgut (1988).units of days as JD [ 2,440,000.

TABLE 8

PSKOVSKII AND PHILLIPS PARAMETERS

*m15(B) b(B) Light Curve Range c Light Curve RangeObject (mag) (mag [100 days]~1) (days) (mag [100 days]~1) (days)

SN 1990N . . . . . . . . . . . . . . . . . . 1.03^ 0.06 11.03 ^ 0.13 11È22 1.380^ 0.004 42È273SN 1991T . . . . . . . . . . . . . . . . . . 0.95^ 0.05 10.21 ^ 0.09 10È20 1.381 ^ 0.007 55È254Leibundgut template . . . . . . 1.10 11.29 . . . 1.676 . . .

244 LIRA ET AL. Vol. 115

FIG. 8.ÈSame as but for SN 1991T. The data appear to fall fromFig. 7,maximum more slowly than the template curves.

Recall that in our work we subtracted a late-time image ofthe region taken around JD 2,449,380 around the super-nova to remove the galactic contribution to the back-ground under the PSF. By doing this, however, we alsoautomatically correct for the echo contamination. Thisassumption is valid provided that the echo magnitude didnot change during the period between the supernovaobservations and the late-time image and that the lightcurve of supernova did not level o† for other reasons, suchas the overproduction of 44Ti or 22Na. Under theseassumptions, the photometry of SN 1991T in andTable 6

should be free of any echo contamination. Indeed,Figure 9the small di†erences between our last points and those ofSchmidt et al. at JD 2,448,750 shown in are consis-Figure 9tent with the echo magnitude cited above.

3.2. Color CurvesThe B[V , B[R, B[I, V [R, V [I, and R[I color

curves for SNe 1990N and 1991T through day 100 areshown in Figures and respectively. The temporal axis10 11,

FIG. 9.ÈLate-time BV light curves for SN 1991T. Photometry fromCTIO (circles) and from KPNO (triangles) are shown. An estimated declinerate for Type Ia supernovae based on the Pskovskii c-parameter is alsoplotted.

FIG. 10.ÈObserved B[V , B[I, and B[R color evolution for SN1990N (open circles) and SN 1991T ( Ðlled circles) as a function of timesince Bmax.

was shifted so that the B maximum corresponds to t \ 0 forboth supernovae. The redder color of SN 1991T withrespect to SN 1990N is evident. The presence of redshiftedNa absorption lines in the spectrum of SN 1991T and thelocation of the supernova in one of the arms of NGC 4527suggest that this object was obscured by dust in its parent

FIG. 11.ÈObserved V [R, V [I, and R[I color evolution of SN1990N (open circles) and SN 1991T ( Ðlled circles) as a function of timesince Bmax.

No. 1, 1998 SNe 1990N AND 1991T 245

FIG. 12.ÈObserved B[V vs. R[ I color-color curves for SN 1990N(open circles) and SN 1991T ( Ðlled circles). The numbers indicate the timeelapsed in days since B maximum.

galaxy. Strong Ca and Na interstellar absorption lines atthe radial velocity of NGC 4527 were observed by Wheeler& Smith and & Roth et(1991) Meyer (1991). Ruiz-Lapuenteal. estimated an excess E(B[V ) D 0.3, assuming a(1992)relationship between the equivalent width of the Na I D lineand E(B[V ). On the other hand, et al. foundPhillips (1992)an excess of 0.13 mag by assuming an intrinsic color B[Vof zero during maximum. The foreground E(B[V )reddening is 0.00^ 0.015 according to & HeilesBurstein(1994).

SN 1990N did not show absorption lines in a low-dispersion spectrum et al. and its loca-(Leibundgut 1991),tion in the outskirts of NGC 4639 suggests that this objectis less reddened than SN 1991T. et al. esti-Saha (1996)mate the mean extinction of the Cepheids asE(V [I) \ 0.04^ 0.06 based on the di†erence in distancemoduli from Cepheid V I period-luminosity relations. Theforeground E(B[V ) reddening is 0.012^ 0.015 accordingto & Heiles It is unfortunate that no high-Burstein (1994).dispersion spectrum of this bright object was made. TheB[V color of 0.03 is consistent with an intrinsic color atmaximum of approximately [0.1 to 0.1 mag for other TypeIa supernovae with low reddenings et al.(Hamuy 1991 ;

& Tammann et al. and sug-Sandage 1993 ; Hamuy 1995)gests has shown that the colorE(B[V ) [ 0.15. Lira (1996)evolution in BV from day 32 to 92 during the nebular phaseis extremely uniform among Type Ia supernovae. In a futurepaper we will use this fact to calibrate the intrinsic colors ofType Ia supernovae at maximum, which, in turn, shouldallow more precise estimates to be made of the host galaxyreddening.

Di†erences in the color evolution of the two supernovaeare better appreciated in the color-color plot shown in

Time along the light curve is indicated by label-Figure 12.ing the points at approximately [10, 0, 10, and 20 daysfrom the maximum. The Ðgure also shows the reddeningvector for a Galactic extinction law & Mathis(Savage

For t [ 10 days, the curves are parallel to the1979).reddening vector. However, the data from t \ [10 tot \ 10 show that the color curve of SN 1991T cannot bematched to that of SN 1990N by a simple dereddeningvector.

4. CONCLUDING REMARKS

SNe 1990N and 1991T are important supernovae. Theywere close enough that distances to the host galaxies can (or

will) be measured by direct techniques with HST . The lightcurves are especially well determined over the full evolution,and, in particular, the evolution before maximum is wellcovered. The light curves of these supernovae have becomestandard templates used in the study of more distant super-novae.

The photometric data presented in this paper show thatSN 1990N was a typical Type Ia event, in that the lightcurves are well Ðtted by the template curve determined by

It also falls in the middle of the range ofLeibundgut (1988).types deÐned by The spectral evolu-*m15 Phillips (1993).

tion of SN 1990N has also been classiÐed as similar to otherprototypical Type Ia supernovae, although the earlyobservations caused it to be claimed as a peculiar object

et al.(Leibundgut 1991).The preliminary reductions of the SN 1990N data in

et al. have been used by et al.Leibundgut (1991) Sandageto estimate a peak absolute magnitude for this super-(1996)

nova. The peak magnitudes cited by Sandage et al. of(B, V ) \ (12.70, 12.61) are D0.07 mag brighter than themore precise results given in Such a small magni-Table 7.tude di†erence will have little e†ect on the measurement ofthe Hubble constant since et al.dH0/H0 B 0.46dm. Hamuy

et al. and et al. have(1996b), Sandage (1996), Riess (1996)used the light curve of this supernova as one of the funda-mental calibrators of the absolute magnitudes of Type Iasupernovae. These absolute magnitudes, coupled with theobserved Hubble diagram from the Cala� n/Tololo survey

et al. have yielded km s~1 Mpc~1.(Hamuy 1996b), H0 D 65Because of its peculiar nature, SN 1991T has been studied

intensively. The peculiarities of this supernova include pre-maximum spectra dominated by iron-group features, a verysmall value, and a visual luminosity larger than other*m15typical Type Ia supernovae, although the derived absolutemagnitudes depend strongly on the di†erent extinctionassumed for the supernova and the distance to the hostgalaxy, NGC 4527 et al.(Filippenko 1992 ; Ruiz-Lapuenteet al. et al. The results of this paper1992 ; Phillips 1992).conÐrm the slow evolutionary rate near maximum and alsoshow that the color curve is signiÐcantly di†erent frommore normal Type Ia supernovae.

This is not to say that SN 1991T is unique, as new eventsof this ““ slow class ÏÏ have been found, such as SNe 1991ag

et al. 1992bc et al. 1995ac(Hamuy 1995), (Maza 1994),et al. and 1997br et al. The(Garnavich 1996), (Qiao 1997b).

evidence suggests that the decline rate of these supernovaeis just the slow end of the peak luminosityÈdecline raterelation for Type Ia supernovae and that this correlationcould be also extended to a spectroscopic sequence (Nugentet al. et al. and et al.1995). Hamuy (1996c) Garnavich

however, have pointed out that the intrinsic lumi-(1996),nosity, spectral features, and colors at maximum light arenot a simple function of the light-curve shape (as measuredby for this bright class of supernovae. For instance,*m15)among supernovae with similarly small values of SNe*m15,1991T, 1995ac, and 1997br had very weak Si II j6355 atmaximum light & Leonard et al.(Filippenko 1995 ; Qiao

while 1992bc had the typical deep spectral features1997a)at maximum light common to most Type Ia events et(Mazaal. Conversely, SN 1995bd had a spectrum similar to1994).SN 1991T at maximum light, but its light curve was wellÐtted by the ““ faster ÏÏ Leibundgut template et al.(Garnavich

It is clearly important to obtain more examples of1996).this class of bright Type Ia supernovae to sort out this issue.

246 LIRA ET AL.

J. M. and M. H. acknowledge support by Cate� dra Presi-dencial de Ciencias 1996È1997. We would like to thank theSpace Telescope Science Institute for access to the DigitizedSky Survey. We thank Peter Garnavich, Eric Olsen, BrianSchmidt, and Gordon Walker for helpful correspondence.This research has made extensive use of the CanadianAstronomy Data Center (Dominion Astrophysical Obser-

vatory, Herzberg Institute of Astrophysics) and the NASAAstrophysics Data System Abstract Service. We would alsolike to thank Brian Marsden and Daniel Green at the IAUCentral Bureau for Astronomical Telegrams for their valu-able notiÐcation service, which allows observers to startobserving supernovae within 24 hours of discovery.

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